Image display devices using organic electroluminescence (EL) elements are well-known as image display devices using current-driven light-emitting elements. An organic EL display device using such self-luminous organic EL elements does not require backlights needed in a liquid crystal display device and is best suited for increasing device thinness. Furthermore, since viewing angle is not restricted, practical application as a next-generation display device is expected. Furthermore, the organic EL elements used in the organic EL display device are different from liquid crystal cells which are controlled according to the voltage applied thereto, in that the luminance of the respective light-emitting elements is controlled according to the value of the current flowing thereto.
In the organic EL display device, the organic EL elements included in the pixels are normally arranged in a matrix. Here, a switching thin film transistor (TFT) is provided in each crosspoint between scanning lines and data lines, the gate electrode of a drive element is connected to the switching TFT, the switching TFT is turned ON through a selected scanning line so as to input a data signal from a data line to the drive element. A display device in which the organic EL element is driven by such a driving element is called an active-matrix organic EL display device.
In an active-matrix organic EL display device, in order to realize high-precision image display, data voltage reflecting a video signal needs to be accurately stored in the pixel circuit. In other words, in the drive element, in order to cause a drive current corresponding to the data voltage to flow to the light-emitting element, the data voltage needs to be accurately stored between the gate and source of the drive element.
Patent Literature (PTL) 1 discloses an image display device having a circuit configuration for accurately storing a data voltage reflecting a video signal into a pixel to realize high-precision image display.
FIG. 13 is a diagram illustrating a circuit configuration of a pixel included in the image display device disclosed in PTL 1. A pixel 510 in the figure includes: switch transistors 511, 512, and 519; an electrostatic storage capacitor 513; a drive transistor 514; an organic EL element 515; a data line 516; scanning lines 517 and 518, a reference power supply line 520; a positive power supply line 521; and a negative power supply line 522. A data voltage storing operation and a light emission operation in the circuit configuration will be described below.
First, a scanning line drive circuit 504 turns ON the switch transistors 511 and 512 in a state where the switch transistor 519 is OFF. Accordingly, a reference voltage VREF of the reference power supply line 520 is applied to an electrode 531, and a data voltage Vdata is applied to the electrode 532 from the data line 516. At this time, since the drain current of the drive transistor 514 does not flow, the organic EL element 515 does not emit light. In this data voltage storing period, voltage drop due to a steady-state current does not occur because only a capacitive load is connected to the reference power supply line 520. Therefore, accurate voltages VREF and Vdata corresponding to the data voltage are respectively stored in the electrode 531 and the electrode 532 of the electrostatic storage capacitor 513.
Next, the scanning line drive circuit 504 turns OFF the switch transistors 511 and 512 to cause a state of non-conduction between the electrode 531 and the reference power supply line 520 and cause a state of non-conduction between the electrode 532 and the data line 516.
Next, the scanning line drive circuit 504 turns ON the switch transistor 519 to cause a state of conduction between the source electrode of the drive transistor 514 and the electrode 532. Here, the electrode 531 is blocked off from the reference power supply line 520, and the electrode 532 is blocked off from the data line 516. Therefore, a gate potential of the drive transistor 514 changes together with the change of the source potential, and (VREF−Vdata), which is the voltage across the electrostatic storage capacitor 513, is applied between the gate and the source, and thus a current corresponding to this (VREF−Vdata) flows in the organic EL element 515, and a light emission operation is executed.
As described above, according to the image display device disclosed in PTL 1, the current flowing in the drive transistor only passes via the light-emitting element at all times, and thus at the time of data voltage storing, the steady-state current does not flow in the reference power supply line and the data line. Accordingly, accurate potentials can be stored in the electrodes on both ends of the electrostatic storage capacitor having a function of storing voltage to be applied between the gate and the source of the drive transistor. Therefore, unevenness in the luminance of light-emitting elements can be suppressed, and high-precision image display reflecting the video signal becomes possible.